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The wavelength dispersion of graphite(002)-monochromated X-ray beams has been determined for a Cu, a Mo and an Rh tube. The observed values for Deltalambda/lambda were 0.03, 0.14 and 0.16, respectively. The severe reduction in monochromaticity as a function of wavelength is determined by the absorption coefficient mu of the monochromator. Mu(monochromator) varies with lambda3. For an Si monochromator with its much larger absorption coefficient, Deltalambda/lambda values of 0.03 were found, regardless of the X-ray tube. This value matches a beam divergence defined by the size of the focus and of the crystal. This holds as long as the monochromator acts as a mirror, i.e. mu(monochromator) is large. In addition to monochromaticity, homogeneity of the X-ray beam is also an important factor. For this aspect the mosaicity of the monochromator is vital. In cases like Si, in which mosaicity is practically absent, the reflected X-ray beam shows an intensity distribution equal to the mass projection of the filament on the anode. Smearing by mosaicity generates a homogeneous beam. This makes a graphite monochromator attractive in spite of its poor performance as a monochromator for lambda < 1 A. This choice means that scan-angle-induced spectral truncation errors are here to stay. These systematic intensity errors can be taken into account after measurement by a software correction based on the real beam spectrum and the applied measuring mode. A spectral modeling routine is proposed, which is applied on the graphite-monochromated Mo Kalpha beam. Both elements in that spectrum, i.e. characteristic alpha1 and alpha2 emission lines and the Bremsstrahlung, were analyzed using the 6,3,18 reflection of Al2O3 (s = 1.2 A(-1)). The spectral information obtained was used to calculate the truncation errors for intensities measured in an omega/2theta scan mode. The results underline the correctness of previous work on the structure of NiSO4*6H2O [Rousseau, Maes & Lenstra (2000). Acta Cryst. A56, 300-307].

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C H 0 ; Mr = 208.1, monoclinic. PZ1/c. a = 3.956(3). b = 7.866(3). c -15.766(4) A, 14 0 2 a = 102.7(3)'. 2 = 2, V = 478.5 i3, F(OO0) = 216. Ox = 1.445 Mg m' 3, Mo-radiation. A = 0.71073 i, u = 0.104 m-', c r y s t a l size 0.15 x 0.2 x 0.15m., R = 0.106, 'i, = 0.102 f o r 718 r e f l e c t i o n s w i t h I > 3 (I ( I ) o u t o f 1024 independent measurements a t room temperature. The redetermination confinns geometrical features given by previous investigations. The poor f i t between observations and model could be shown n o t t o be caused by e x t i n c t i o n e f f e c t s .0

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